1、創建線程
2、在線程開始運行的時候，通過Looper.prepare() 創建消息隊列。中間涉及到ThreadLocal，保證了該線程只能有一個Looper和一個消息隊列。
3、然後Looper.loop(); 開始了無限循環模式，不斷從MessageQueue中獲取消息，然後調用Message的target 的dispatchMessage 處理消息。MessageQueue也在不斷的循環等待來新的消息，纔會返回到Looper中。
4、Message創建，可以從緩存中獲取已經處理的消息，或者創建新的對象
5、Handle 發送Message， 賦予Message的target值。 並且調用MessageQueue 的enqueueMessage 方法，按照Message.when順序插入到隊列之中。等待MessageQueue的next中無限循環獲取。
6、Looper.loop() 這個無限循環終於拿到了MessageQueue 返回的Message了， 調用Message.target.dispatchmessage方法， 也就是Handle的dispatchmessage 開始處理消息。
7、消息處理完成，Message.recycleUnchecked 放入緩存池，等待下次使用。
一、消息分發的關鍵類
Handler： 消息句柄，實現消息的發送和接收
Message: 消息對象
MessageQueue: 消息隊列，負責消息的管理
Looper： 負責從MessageQueue中不斷取出Message對象，實現分發
ThreadLocal: 保證了Looper在一個線程中只存在一個實例。
二、從IntentService分析消息分發
從IntentService源碼可以看出，內部主要靠 HandlerThread 實現線程的創建，和ServiceHandler實現任務在線程執行。
@Override public void onCreate() { super.onCreate(); HandlerThread thread = new HandlerThread("IntentService[" + mName + "]"); thread.start(); //創建線程和消息隊列 mServiceLooper = thread.getLooper(); mServiceHandler = new ServiceHandler(mServiceLooper); //創建Handler } @Override public void onStart(@Nullable Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); } @WorkerThread protected abstract void onHandleIntent(@Nullable Intent intent); // 處理消息1、HandlerThread是如何創建消息隊列的？
HandlerThread 是一個包含 Looper 的 Thread，擁有自己的消息隊列。我們可以直接使用這個 Looper 創建 Handler。
HandlerThread其實就是Thread的子類，最終運行會調用到run方法。
@Override public void run() { mTid = Process.myTid(); Looper.prepare(); //構建Looper synchronized (this) { mLooper = Looper.myLooper(); //獲取Looper notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); //開始循環Loop mTid = -1; } 讓我們跟進 Looper.prepare()
public static void prepare() { prepare(true); } private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); //一個線程只能創建一個Looper實例 } sThreadLocal.set(new Looper(quitAllowed)); } private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }可以看到創建了一個新的Looer對象，放到了sThreadLocal 這個靜態變量之中。
疑問：爲什麼要寫成靜態呢？
回答：爲了存儲Looper對象，保證一個線程只能創建一個Looper實例。
繼續深入，看看ThreadLocal 的set方法
/** * Sets the current thread's copy of this thread-local variable * to the specified value. Most subclasses will have no need to * override this method, relying solely on the {@link #initialValue} * method to set the values of thread-locals. * * @param value the value to be stored in the current thread's copy of * this thread-local. */ public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); } void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }大概意思可以看出，ThreadLocal其實是依據當前Thread線程，取出當前線程中，以ThreadLocal爲Key存儲的值。這裏就是Looper 了。
爲此，我們看看ThreadLocalMap
/** * ThreadLocalMap is a customized hash map suitable only for * maintaining thread local values. No operations are exported * outside of the ThreadLocal class. The class is package private to * allow declaration of fields in class Thread. To help deal with * very large and long-lived usages, the hash table entries use * WeakReferences for keys. However, since reference queues are not * used, stale entries are guaranteed to be removed only when * the table starts running out of space. */ static class ThreadLocalMap {.... //從這段註釋中，可以理解ThreadLocalMap是一個自定義的HashMap，僅適用於存儲線程的本地值。 /** * Construct a new map initially containing (firstKey, firstValue). * ThreadLocalMaps are constructed lazily, so we only create * one when we have at least one entry to put in it. */ ThreadLocalMap(ThreadLocal firstKey, Object firstValue) { table = new Entry[INITIAL_CAPACITY]; int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); table[i] = new Entry(firstKey, firstValue); size = 1; setThreshold(INITIAL_CAPACITY); }好，我們繼續回到HandlerThread中， 繼續run方法
synchronized (this) { mLooper = Looper.myLooper(); //獲取Looper notifyAll(); }看看Looper.myLooper()做了什麼？
/** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ public static @Nullable Looper myLooper() { return sThreadLocal.get(); } // =====ThreadLocal.get() public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) return (T)e.value; } return setInitialValue(); } /** * Variant of set() to establish initialValue. Used instead * of set() in case user has overridden the set() method. * * @return the initial value */ private T setInitialValue() { T value = initialValue(); //返回null Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); return value; } protected T initialValue() { return null; } 從上面可以看出，Looper.myLooper() 就是爲了取出之前prepare（）時候，依據當前線程創建的Looper對象。
再繼續往下：Looper.loop
public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block，循環等待消息的來臨 if (msg == null) { // No message indicates that the message queue is quitting. return; }//... try { msg.target.dispatchMessage(msg); //關鍵來了， 這裏處理了消息的分發 } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } }//... // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked();//放入緩存池 } }通過上訴代碼可以看到：msg.target.dispatchMessage(msg) 實現了最終的消息分發。
有個疑問？
如何實現的消息的等待，看源碼，一旦返回消息爲空，就退出了。帶着這個疑問，我們看看MessageQueue的next方法：
Message next() { // Return here if the message loop has already quit and been disposed. // This can happen if the application tries to restart a looper after quit // which is not supported. final long ptr = mPtr; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { //關鍵，一直循環等待，直到條件滿足才返回！ if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); } if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }從上訴代碼發現，當沒有消息的時候，一直for空轉循環等待。直到mQuitting爲true的時候，纔會退出。所以當我們自定義線程使用Looper，必須主動調用退出。
好，現在問題來了:
message 的 target是如何賦值的呢？
現在讓我們再回到IntentService中，看看serviceHandle的構建：
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]"); //創建線程、Looper thread.start(); //Looper開始等待 mServiceLooper = thread.getLooper(); mServiceHandler = new ServiceHandler(mServiceLooper); // 這裏調用了Handle的構造方法首先看看Handler的構造函數：
public Handler(Looper looper) { this(looper, null, false); } /**最終都會調用到以下構造函數 * @hide */ public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }從以上代碼可以看出，Handler 屬性mLooper，mQueue，mCallback，mAsynchronous=false。
其中：mCallback 顧名思義，就是處理消息的回調，至於mAsynchronous=false ，常用幾個構造函數都是false，暫不考慮。
前面已經知道Looper、MessageQueue消息隊列。
繼續回到IntentService
@Override public void onStart(@Nullable Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); }問題：
1、消息是如何創建的？
2、sendMessage 是如何將消息放入消息隊列的？
消息是如何創建的
查看Handler obtainMessage(）源碼：
/** * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this). * If you don't want that facility, just call Message.obtain() instead. */ public final Message obtainMessage() { return Message.obtain(this); } #=====Message public static Message obtain(Handler h) { Message m = obtain(); m.target = h; return m; } /** * Return a new Message instance from the global pool. Allows us to * avoid allocating new objects in many cases. */ public static Message obtain() { synchronized (sPoolSync) { if (sPool != null) { Message m = sPool; sPool = m.next; m.next = null; m.flags = 0; // clear in-use flag sPoolSize--; return m; } } return new Message(); }可以看到，最終從全局緩存池中獲取，沒有就創建一個。
問題：什麼時候會將Message放到緩存池?
前面Looper.loop 方法中可以看到，在消息分發處理完成後，就放入了緩存池。
/** * Recycles a Message that may be in-use. * Used internally by the MessageQueue and Looper when disposing of queued Messages. */ void recycleUnchecked() { // Mark the message as in use while it remains in the recycled object pool. // Clear out all other details. flags = FLAG_IN_USE; what = 0; arg1 = 0; arg2 = 0; obj = null; replyTo = null; sendingUid = -1; when = 0; target = null; callback = null; data = null; synchronized (sPoolSync) { if (sPoolSize < MAX_POOL_SIZE) { next = sPool; sPool = this; sPoolSize++; } } }如何將消息放入消息隊列
handle.sendMessage
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); } private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; // 指定了target爲當前發送的Handler if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }根據以上代碼層層跟進，最終發現了我們的目標：MessageQueue。enqueueMessage 將實現消息放入到隊列之中。
boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { //隊列爲空，放在隊列的最前面 // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don't have to wake // up the event queue unless there is a barrier at the head of the queue // and the message is the earliest asynchronous message in the queue. needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { //隊列不爲空，按照發生時間先後順序，插入到隊列之中 prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }消息如何被處理
之前在Looper中已經看到，msg的target 也就是Handler處理了消息。
/** * Handle system messages here. */ public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }等等，mCallback是我們在Handler構造函數傳遞的，handleMessage， 也是處理消息的，msg.callback又是什麼呢？
再次進入到Message類中，發現msg.callback是一個Runnable， 做個假設，是不是Hander post的的這個Runnable呢？ 看看源碼：
/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }果然，上訴代碼驗證了我們的猜想。所以Handle在分發處理消息的時候，優先處理這個runnable， 然後再處理我們傳入的callback，最後纔是handleMessage。
綜上，整個消息的分發流程就分析完成。總結以下
三、總結
1、創建線程
2、在線程開始運行的時候，通過Looper.prepare() 創建消息隊列。中間涉及到ThreadLocal，保證了該線程只能有一個Looper和一個消息隊列。
3、然後Looper.loop(); 開始了無限循環模式，不斷從MessageQueue中獲取消息，然後調用Message的target 的dispatchMessage 處理消息。MessageQueue也在不斷的循環等待來新的消息，纔會返回到Looper中。
4、Message創建，可以從緩存中獲取已經處理的消息，或者創建新的對象
5、Handle 發送Message， 賦予Message的target值。 並且調用MessageQueue 的enqueueMessage 方法，按照Message.when順序插入到隊列之中。等待MessageQueue的next中無限循環獲取。
6、Looper.loop() 這個無限循環終於拿到了MessageQueue 返回的Message了， 調用Message.target.dispatchmessage方法， 也就是Handle的dispatchmessage 開始處理消息。
7、消息處理完成，Message.recycleUnchecked 放入緩存池，等待下次使用。